1. Field of the Invention
The present invention relates to an image forming apparatus and a power control method.
2. Description of the Related Art
A typical image forming apparatus, such as a printer, a copier, and a facsimile, includes a fixing unit. The fixing unit melts a toner image formed on a recording medium (such as a paper and an OHP transparency) by heat generated by a heating unit, and applies a pressure to the recording medium, to fix the toner image on the recording medium. The heating unit includes a halogen heater made by placing tungsten filaments in a glass tube filled with inert gas.
Heating of the halogen heater is controlled by changing the time during which power is supplied from a commercial power supply unit with a power control element such as a triac. A general power control method includes a phase control method and an on/off control method. In the phase control method, a triac is turned on after a predetermined time from a zero-cross point of an alternating current (AC) voltage applied from a commercial power supply unit, and thereafter, the power is turned off at the timing when the polarity of the AC power voltage applied to a heating unit is reversed due to the characteristics of the triac, thereby controlling the time from the zero-cross point to the turning-on of the triac. In the on/off control method, a power control element is turned on/off every half cycle or more of a frequency of a commercial power supply unit.
In a soft start and a soft stop, a phase angle is gradually changed when starting/stopping supplying power to a halogen heater. The soft start and the soft stop prevent a large current from flowing into the halogen heater at the instant of turning on/off the halogen heater by gradually increasing/decreasing the on-duty ratio at the time of starting/stopping supplying power to the halogen heater. Accordingly, influence on other devices due to a harmonic current and a power voltage variation is suppressed. In both of the phase control method and the on/off control method, the halogen heater is controlled based on a zero-cross timing of the frequency of the power supply unit.
An image forming apparatus having a high throughput often employs a fixing roller with large heat capacity at a heating unit so that a temperature drop of the fixing roller is prevented at the time of image formation. However, because such fixing roller needs about a few minutes to reach a predetermined temperature to be ready for use, a waiting time for outputting a printed sheet is relatively long. When a fixing roller with smaller heat capacity is used to shorten the temperature rising time of the fixing roller, a temperature drop of the fixing roller may occur at the time of image formation.
Using a power source of 200 V can increase power capacity of a heating unit, such as a halogen heater, or a power supplied to the fixing roller, so that the temperature rising time of the fixing roller can be shortened. However, in some countries or regions, a typical commercial power supply unit in common offices is 100 V×15 A, and a 200 V power supply is available only with a special installation. Thus, expecting a voltage higher than 100 V is not realistic.
Japanese Patent Application Laid-open Nos. 2003-244359 and 2003-323085 disclose technologies in which a plurality of power cords are provided, and the power is supplied from a plurality of commercial power supply units through the power cords, thereby supplying the power over 100 V×15 A to a load.
However, in the above technologies, because both reference phases (zero-cross points) and frequencies of AC powers supplied through the power cords need to be the same, if the reference phases or the frequencies of the supplied AC powers are not the same, the power control methods in the above technologies cannot be performed properly. For example, when the power is supplied through two power cords, if the frequencies of the AC powers supplied through the power cords are 50 Hz and 60 Hz, respectively, or if one of the AC power supply units is a commercial power supply unit while the other one is a different type of power supply unit, such as a private power generator having a different frequency, the zero-cross timings become different from each other. Therefore, because the phase of either one of the AC power supply units cannot be controlled properly, a harmonic current or a power voltage variation may occur due to the interference of both frequencies, which may result in disadvantageously affecting other devices.